The long-term goal of this project is to understand the process of speciation through genetic analysis of closely-related species. We will use classical genetic methods to reveal the numbers, locations, and effects of genes causing mating discrimination between Drosophila species as well as of genes producing interspecific differences in cuticular hydrocarbons, which serve as female mating pheromones. Applying such techniques to a variety of Drosophila groups will help us answer the following questions: 1. Are mating discrimination and cuticular-hydrocarbon differences among closely-related species due to only one or a few genes of large effect, or are several to many genes involved? 2. Where in the genome are the loci causing these forms of reproductive isolation? Are they located in similar regions among different species pairs of a group, implying genetic "hotspots" of speciation? 3. Does sexual isolation among males and females have a similar genetic basis, or are different genes involved in the two sexes? 4. Are there consistent patterns to the genetics of sexual isolation and pheromonal hydrocarbons that apply across several groups? 5. Are the genes involved in mating discrimination concentrated on the sex chromosomes, like those causing hybrid sterility and inviability? 6. What role do cuticular hydrocarbons play in sexual isolation between species? Although sexual isolation is one of the most potent causes of speciation, we know virtually nothing about its genetic basis. These studies will provide data important for understanding the origin of species and in motivating new theories of speciation. The fine-structure mapping of genes causing reproductive isolation will eventually facilitate their molecular identification, which is required for a complete understanding of speciation. Our results may be relevant to understanding the origin of species in other animals, including humans.